Estrus
by Andromeda Prime
Summary: TF: Prime. It was medical fact that there were two reasons why a Cybertronian's gestation chamber would expand. Rated M for slash, smut, mechpreg references, and heat cycles. Giftfic for the lovely Kem, AKA cousinswar. COMPLETE.


**Very belated gift-fic for **Kem **(**cousinswar **on this site, **farfalleh **on Tumblr). I came up with this little headcanon thing a while back, based off a gif I saw on Tumblr where Optimus was about to kill Megatron in "One Shall Fall" and it showed a slight curve to his abdomen, and she wanted a fic with it.**

**Here you go darling! Sorry it's so late :)**

.-.-.

How long had it been since they'd managed to be in the same room together, for longer than mere moments, and not try to kill each other?

Far, far too long.

Well, there had been the little rendezvous at the tiny and rather…_quaint_…little Autobot base. But even then, Megatron could tell everyone was holding themselves back from releasing their weapons and taking him down. Especially him.

Megatron watched from the central monitoring room as Orion saved the rest of his work and shut down his computer terminal, retiring for the night. His optics shifted to other screens, watching as the data clerk made his way through the corridors to his room. The viewing angle the camera gave him let him see Orion from sideview as the data clerk tapped the security code into the console and let him into his quarters.

Heat flashed through the Decepticon warlord's frame at the sight of that soft swell to Orion's upper abdomen.

.-.-.

It was medical fact that there were two reasons why a Cybertronian's gestation chamber would expand.

The first was that they were about to hit their heat cycle. The chamber would become swollen with fluids that, in the event of sparking, would cushion and nourish the sparkling as they grew. When the heat cycle passed and if no sparkling was introduced, half of the gestative fluids would be converted back into energon and spread throughout the Cybertronian's frame again, until the next heat cycle hit.

The second would be because there was already a sparkling inside of the chamber, and the little one had grown large enough to cause the abdominal plating to expand outward.

As he'd laid on Earth, looking above him with wide lilac optics as Optimus Prime unsheathed his blade and made to strike him down, he noticed it. A faint, soft curve to his now-nemesis, once-lover's, upper midsection. He likely wouldn't have seen it if it weren't for the lightning that flashed across the sky at that moment, turning night into day.

In that moment he knew the Prime was either in heat, or about to enter the cycle. There was no way the other Autobots would have allowed their precious Prime to endanger any little life he might carry. For the briefest moment, when he'd gained the upper hand over the Prime again and was about to deliver what would have been the final, fatal blow, he considered taking the Autobot Commander back to the _Nemesis_ instead. The thought of taking the other mech, fragging him in every which way possible until the Prime was swollen with his sparkling, sent a delightful shiver through his spinal struts.

Then the rest of the Autobots came to their Prime's rescue. And it didn't seem he'd have much of a chance to exploit the Prime's upcoming condition.

Until wide blue optics stared back at him from the core, and a voice that was the epitome of innocence asked for Megatronus.

Megatron had been displeased when he brought Orion Pax onboard and the little mech didn't show any signs of being in his heat cycle. Orion did not hastily excuse himself to be alone. The air around him did not become heated, and he didn't tremble whenever someone else touched him. Orion was stripped of his Autobot insignia, replaced with a Decepticon badge. He was shown around the ship.

Nothing.

The Decepticon warlord strode through the warship's corridors, heading for Orion's assigned quarters. At the close of each day, Megatron paid the young data clerk a visit to inquire about his day. More often than not both of them would end up talking about Cybertron, days, _eons_, gone by. Megatron had few good memories to smile about, to cherish. His life on Cybertron had mostly been all dark and dreary, the only things he could seem to remember for most of it being the mines he labored in.

Then came the gladiatorial matches. Then came the little clerk named Orion Pax.

And his life cycle had finally found meaning.

He let his lipplates twitch upward into a tiny grin as he would hear the data clerk go on and on about how much he loved the Hall, how much he looked forward to returning to Cybertron, restoring it. And maybe then he'd finally be able to go to Six Lasers. Just maybe.

How Megatron wanted to give him that.

The quarters Orion had taken over were formerly those of Starscream. They were second-largest in size, smaller than only Megatron's and the exact same size as Soundwave's. Megatron rapped his digits on the locked doors and called for the clerk.

"Orion."

Silence. Megatron waited for a few more moments before rapping at the doors again, increasing the volume of his voice. "Orion!"

Nothing for a brief moment, then he heard a faint sound. A low moan.

Megatron wasn't entirely sure what he thought was going on, but he tapped the override code in the security panel. The doors slid open, revealing a dark receiving room. He walked through, the doors closing behind him, and headed for the adjoining room. The berthroom. His optics were still adjusting to the blackness, but when they did he saw a very welcome sight.

Orion was on the berth, sitting with his back strut against the wall. His helm was arched back and his optics shuttered. One servo braced himself against the berth's surface, and another was positioned at the apex of his thighs. Slim digits teased the rim of his open and bared valve, inching close as if to penetrate himself but then drawing back. Was Orion Pax…ashamed? Scared?

The Decepticon warlord smirked to himself, watching the data clerk's faceplates change and align themselves into a number of expressions. Arousal. Fear. Want. A brief moment of self-loathing. Lust. Finally, _finally_, after what seemed to be the entire night Orion worked up his courage and hesitantly pressed a digit into his valve. The expression that passed over his faceplates sent the most wonderful, delightful shivers up Megatron's spinal strut.

He wasted no time in reaching forward and tracing a sharp digit along part of the other mech's valve rim for a quick moment. His digit plunged into the spasming valve the same moment that Orion's optics snapped open and bright blue optics met his crimson ones.

"M-Megatron! I-"

The warlord had already climbed onto the berth and leaned over Orion Pax, his heavier frame hovering over the librarian's lither frame. He captured the red and blue mech's lipplates in a passionate kiss, nipping at the lower lipplate as he murmured, "You're in your heat cycle, aren't you Orion?"

A quiet, tense moment filled with the sounds of their engines attempting to cool down. Then Orion nodded desperately, returning the kiss with equal fervor. "Just today. The cycle hit, Megatron. Today."

The Decepticon warlord growled lustily, processor producing images of him taking the former Prime and data clerk here, on this berth, against the wall, on the floor. Then, when Orion Pax was swollen with his transfluids and was unable to move without some degree of discomfort, he would join their sparks.

And then Orion Pax would be swollen with sparkling. His heir.

Megatron smirked into the kiss as he slowly inserted another digit into the smaller mech's valve, savoring Orion's reaction. The librarian moaned and squirmed underneath him, panting and pleading, "Please, Megatron. _Please_."

Nipping at the red mech's bottom lipplate again, the warlord growled. "You want me to help you through your cycle, Orion?"

"_Yes."_

Megatron moved his helm, trailing down from Orion's lipplates to those delicious and dark neck cables. He latched onto them, biting down on them, growling, "Do you want me to take you? Do you want me to take you on this berth, right now?"

Orion nodded fervently, shifting his hips to try and draw more of those pointed digits into his valve, moaning.

"You want me to take you hard and fast, so many times that I fill you up with my fluids and you can hardly walk?"

Orion's optics pleaded with Megatron. The warlord chuckled and growled as his interfacing panel snicked back, revealing his erect cable. "And after that, do you want me to pin you down, reveal your spark and my spark, and join them until we overload? Would that make you happy, Orion? Would that make you feel better?" Megatron grabbed one of the other mech's legs and hooked it around his waist, crimson optics darkened to a shade of deep red. "Would you like that? Would you like to be carrying my sparkling? Your chamber swollen with my seed?"

The data clerk moaned, optics flaring. Servos reached up and hooked themselves onto Megatron's shoulder armor, grasping as he gasped and gasped. "Yes, _please_. Megatron, please!"

A brief kiss. A smirk. And Megatron only growled, "As you wish it, Orion."

Then he surged forward. And smiled when Orion gasped in pleasure.

Megatron took Orion Pax on the berth. Twice. Once with Megatron over Orion, and the next with both laying on their sides, Megatron's front to Orion's back. Then the librarian surprised him with a show of strength and he rolled them both off the berth and onto the hard floor below. Megatron grunted in pain at the contact, but the pain was quickly forgotten as Orion began rolling his hips and giving him that smirk that made him want to kiss him.

Then they found the strength to stand. Well, Megatron did. He grabbed Orion and held him against a wall, wrapping his legs around his hips and thrusting wildly.

And here they were, in Orion's washracks, cleansing solvent pouring down Megatron's back as he held Orion against the wall here, servos all over each other, kisses passionate and moans breathless. Orion stared at Megatron with half-lidded optics and trailed his digits down the center of the silver mech's chassis. Megatron shuddered and raised a servo to grip at Orion's. The data clerk raised his helm to look at the warlord in the optics. "Come on," Orion whispered, optics darkened with arousal, want, lust.

Megatron only nodded for a moment before he opened his chassis. A darkened blue spark revealed itself, scarred by eons of war and evils that only he knew of and that Orion didn't.

Orion didn't seem repulsed by the gloomy color. The librarian only reached for it and stroked the energy field with his digits, and whispered, "You've changed much, Megatron."

In response the warlord placed his servo in the center of Orion's chassis, a silent plea for them to open. Orion looked at him for a brief moment before obliging him, and his chassis plates split open to reveal a vivid blue spark, still so untainted and loving after all these eons of war. There had been virtually no change since the last time they'd joined sparks, had been intimate.

How easy would it be, to just stroke the other mech's life force, send him into pleasure, then without warning to snuff the spark?

Megatron already had his digits in Orion's spark field, and the data clerk was twitching and moaning.

He would ponder that question another time, when he did not have a primal need to take this bot, mate with him, fill him with his seed, his sparkling. He lined up both their chassis, and penetrated the smaller mech's valve with his spike again at the same time their sparks joined and merged together, sparks only grazing by each other, not deep enough to form a bond. Overload claimed them both within moments, Megatron roaring his release and Orion Pax moaning in pleasure.

The former Autobot leader's chamber would no longer only be swollen with fluids.

.-.-.

**Don't have any plans of continuing this. You could actually consider this as a sort of prequel to the events of "Amnesiac". You take your pick.**

**Reviews are loved! And I hope you loved it, **Kem**!**


End file.
